1. Field of the Invention
The present invention relates generally to an image processing apparatus and method, and in particular, to an image processing apparatus and method for preventing degradation of image quality occurring when a bit format of an image is converted.
2. Description of the Related Art
In general, accompanying a significant increase of performance of mobile communication terminals such as cellular phones, mobile communication terminals, which can realize various multimedia functions, have become widely available on the market. In particular, quality of content or images for performing the various multimedia functions supported by the mobile communication terminals have been developed to a higher quality. Although a performance level of a menu screen or a screen for performing a multimedia function in such a mobile communication terminal is almost similar to that of a personal computer (PC) or a television (TV), a technical gap exists in at least a display unit for displaying on such a screen.
A liquid crystal display (LCD) is mainly used for display units of the mobile communication terminals, and color reproduction performance of the display units is an 18-bit color level or less. In particular, an interface of a back-end chip for driving the display units of the mobile communication terminals is a 16-bit level. When colors of an image are displayed by such a display unit, the number of bits of an interface is very important. In general, since 24-bit true color are realized in a PC or a digital TV, the number of colors, which can be used for an image, is 16,700,000 (=224).
However, a display unit of a mobile communication terminal can identify only 65,000 (=216) colors, which is much less than the 16,700,000 (=224) colors that can be identified by a PC or a digital TV. When content to be displayed by a mobile communication terminal is produced using a PC, a 24-bit image is edited, and when the edited 24-bit image is stored in the mobile communication terminal, the 24-bit image must be converted to an image having color resolution supportable by a display unit of the mobile communication terminal. If the display unit of the mobile communication terminal supports 16-bit color, the mobile communication terminal stores the image by converting the 24-bit image edited using the PC to a 16-bit image.
Color resolution of an LCD included in early mobile communication terminals was 256 colors, and bitmap images having a quality corresponding to 256 colors were used for a graphic user interface (GUI). Even though performance of an LCD interface of a mobile communication terminal more than doubled due to a quick increase of a hardware developing speed, a content upgrading speed was slow, and thus, content having color resolution much less than 16 bits was provided to the mobile communication terminal as illustrated in diagram (a) of FIG. 1. When color resolution used for an image was less than 16 bits, even if the image was edited using 24-bit color in a PC and converted to a 16-bit image, there was no problem in the side of the mobile communication terminal.
However, according to a quick development of a GUI of a mobile communication terminal, an application range has widened, and thus, color resolution used to edit an image is more than 16 bits. Thus, when a bit format of an image is converted in a PC, that is, if an image is converted to an image having color resolution supportable by a mobile communication terminal supporting 16-bit color, since the mobile communication terminal cannot express all colors used in 24-bit color, the converted image is created using a method of mapping the nearest colors among the 16,700,000 colors to a representative color in order to express the 65,000 colors from the 24-bit raw image.
As illustrated in diagram (b) of FIG. 1, when a photograph or a high quality image corresponding to a photograph is edited in a PC and converted to an image to be used in a mobile communication terminal, the number of lost colors is significant, thereby occurring profound degradation of image quality. In more detail, as illustrated in FIG. 2, colors corresponding to pixel values 0˜255 in a 24-bit image can be mapped to a color corresponding to a pixel value 0 in a 16-bit image, and color corresponding to pixel values 256˜511 in the 24-bit image can be mapped to a color corresponding to a pixel value 1 in the 16-bit image. That is, although colors corresponding to pixel values 512 and 767 are different colors in a 24-bit raw image, the colors are mapped to the same color corresponding to a pixel value 2 in a 16-bit image after performing an image bit-format conversion process.
As described above, if a 24-bit image is converted to a 16-bit image, degradation of image quality is profound. FIG. 3 illustrates degradation of image quality occurring when a 24-bit image is converted to a 16-bit image. While an image illustrated in diagram (a′) of FIG. 3, which is obtained by magnifying a portion of a 24-bit image illustrated in diagram (a) of FIG. 3, is smoothly gradated, since a 16-bit image illustrated in diagram (b) of FIG. 3, which is obtained by converting the image illustrated in diagram (a) of FIG. 3, cannot use all of 24-bit colors, colors close to each representative color are expressed as the representative colors in the 16-bit image illustrated in diagram (b) of FIG. 3, thereby occurring discontinuous gradation. While colors are distributed all over the range in diagram (a) of FIG. 4A, which illustrates pixel distribution of the image illustrated in diagram (a′) of FIG. 3, colors are distributed on sub-sampled representative values not all over the range in FIG. 4B, which illustrates pixel distribution of an image illustrated in diagram (b′) of FIG. 3.
As described above, according to the prior art, a photograph or a high quality image corresponding to a photograph, i.e., an image in which more than 65,000 colors are used, is edited, converted to an image suitable for a mobile communication terminal, and transmitted to the mobile communication terminal. Since all colors used in an image expressed using more than 65,000 colors cannot be used as they are, the image is mapped to and expressed by using representative colors in an image bit-format conversion process. Thus, in the process of converting a raw image to an image suitable for a mobile communication terminal, which is performed in a PC, degradation of image quality is profound. In order to effectively implement a GUI used in a mobile communication terminal, a method for minimizing the degradation of image quality when an image is converted to an image suitable for a mobile communication terminal in an external device, such as a PC, is required. In addition, a method for performing conversion of a downloaded or stored image in a mobile communication terminal without converting a raw image to an image suitable for the mobile communication terminal is required.